Optical systems have to fulfil increasing demands with respect to their performance. For example, the size of optical components of telescopes steadily increases in order to collect a limited number of photons originating from far distant objects. Slightest deviations of optical components from their predefined surface forms result in a reduction of optical capabilities of the telescopes or generally of optical systems.
On the other hand, microscopes have for example to resolve objects having a very low contrast compared to their surroundings. Moreover, it is requested that projection systems of lithography devices resolve smaller and smaller structures.
Integrated circuits (ICs) are another kind of devices which has to also fulfil steadily increasing demands. According to Moore's law the minimum dimension of structural elements generated on wafers in order to fabricate ICs continuously shrinks.
In the following, the increasing demand on optical systems is exemplified for projection systems, in particular photolithographic projection exposure systems.
As a result of the constantly increasing integration density in the semiconductor industry, photolithographic projection exposure systems and photolithographic masks have to project smaller and smaller structures onto the photoresist arranged on a wafer. In order to fulfil this demand, the exposure wavelength of projection exposure systems has been shifted from the near ultraviolet across the mean ultraviolet into the deep ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers.
As a consequence, the manufacturing of optical components of projection exposure systems is becoming more and more complex, and thus more and more expensive as well. In the future, projection exposure systems will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (e.g. in the range of 10 nm-15 nm).
In such a wavelength range, EUV optical elements, as for example mirrors or photolithographic masks, have to fulfil highest demands with respect to planarity, pureness and temperature stability. The tolerable deviation of the substrates of these optical elements regarding the planarity is only a portion of a wavelength of the exposure wavelength in order to not significantly disturb the phase front of the electromagnetic wave reflected from a multi-layer structure arranged on a surface of the substrate. Larger deviations of the planarity of the substrate of EUV mirrors and masks may lead to variations of the optical intensity distribution in the photoresist due to a (partial) constructive or destructive addition of the wave front in the photoresist of the wafer. At the further processing of the wafer, the variations of the optical intensity may result in the fabrication of defective semiconductor devices as for examples ICs.
EUV substrates as supplied from the manufacturer may not even fulfil the planarity condition for EUV mirrors and masks. Further, the manufacturing process of mirrors and masks which forms a multilayer structure and fine patterns on one surface of the multilayer structure, respectively, may even deteriorate the planarity of the substrate.
The applications U.S. Ser. No. 13/179,799, filed on Jul. 11, 2011, and U.S. Ser. No. 13/252,480, filed on Oct. 4, 2011, of the applicant describe a two-dimensional model to correct registration errors, transmission errors of transmissive masks, and overlay errors of different masks. They are hereby incorporated herein in their entirety by reference.
The U.S. application Ser. No. 13/084,991, filed on Apr. 12, 2011, of the applicant discloses a method for locally correcting a substrate thickness defect of an EUV mask by the generation of color centers inside the substrate surface. This document is also incorporated herein in its entirety by reference.
Furthermore, a curvature of the substrate of a photolithographic mask may also lead to imaging errors of an EUV mask. The U.S. 2008/0 032 206 A1 describes a method to improve the planarity of a manufactured photolithographic mask. To adjust a curvature of the substrate or to smooth an unevenness of the substrate, this document proposes forming expansion stress and/or compaction stress generation portions in a predetermined region of the substrate which includes the curved region. The expansion stress and compaction stress generation portions are generated by focussing femtosecond laser pulses in this region which locally modify the bonding state of the substrate.
The documents outlined above provide approximations for the correction of defects of existing optical components. However, the ever increasing defect correction requirements of future high performance optical components are still not met.
Moreover it is observed that wafers sometimes bend during the manufacturing process so that it is difficult to clamp the wafers with a vacuum chuck. The bending seems to be caused by stress which is introduced into the wafer during the manufacturing process of ICs. Presently, wafer bending occurring during the processing of the wafer can only be lowered by reducing the stress induced by various processing steps of the manufacturing process. For this purpose, the processing steps have to be modified. This is an involved task due to the highly complex sequence of processing steps required to fabricate modern ICs. Additionally, it may also be necessary to tolerate performances losses in the electrical function of the fabricated ICs.
As an alternative approach, layers are at the moment developed which can be applied to the rear surface of a wafer and which reduce the bending effect of various processing steps. Adding an additional layer to the rear wafer side would introduce further processing step in an already involved manufacturing process of modern ICs.